The present invention relates to a control circuit for controlling operation of a three-phase motor. In particular, the present invention is a hard drive spindle motor controller which eliminates reverse current into the power supply during commutation.
Spindle motors used in hard disc drives of computers are typically three-phase motors having three terminals. When a computer is first started, the hard drive spindle motor is typically spun up from a stopped condition by a motor controller circuit using six-state commutation, with peak current limiting and a relatively high motor current. To spin up the motor as quickly as possible, the start current is set as high as the hard drive power supply can deliver. In some systems, when the spindle motor approaches its normal operating speed, control of the spindle motor is transferred to a second motor controller circuit which provides sinusoidal drive to the three terminals using pulse width modulation.
Peak current limiting during spin up operates by allowing the current through the motor to ramp up to a programmed maximum as one terminal (through its motor driver) is tied to the operating high voltage and another terminal (through its motor driver) is tied through a sense resistor to the operating low voltage. While the terminals are tied or connected in this fashion, the motor current ramps up through the motor inductance and resistance between the two terminals. The motor current is measured by measuring the voltage across the sense resistor. When the current through the motor reaches the peak current, the two terminals (which had been connected to a high voltage and a low voltage) are connected to the same voltage. This allows the motor current to decay from the peak current value. Since the motor coil acts as an inductor, the motor current does not drop to zero instantaneously. Instead, the current decays at an approximately linear rate. Typically, the off time during which both terminals are connected to the same voltage is not long enough for the motor current to decay completely to zero. When the two motor terminals are switched back to the state where one terminal is at high voltage and the other is at low voltage, the motor current ramps up from that point until the peak current is again reached. Each time that peak current is reached, the two terminals are connected to the same voltage and motor current is allowed to decay.
During the spin up phase of operation, the current flowing through the motor from the high operating voltage to the low operating voltage is seen across the sense resistor as a series of short, nearly rectangular pulses. The current is only drawn from the power supply (or a supply bypass capacitor in parallel with the power supply) when the two terminals are in the state where they are connected to different voltages. When the two terminals are connected to different voltages, the current flow is from the power supply into the high voltage terminal, through the motor, and out the low voltage terminal of the motor and through the sense resistor to ground. When the two terminals are both connected to the high operating voltage, the current flow is from the power supply node into the high voltage terminal, through the motor, and back to the power supply node. Therefore, no motor current is drawn from the power supply or bypass capacitor. Current does flow from the power supply into the bypass capacitor during this time. Thus, the current draw is represented by the voltage pulses across the sense resistor. The supply bypass capacitor tends to filter these narrow supply current pulses, which reduces the peak load current seen by the power supply. An average motor current is set by the alternation between the two states in which a pair of motor terminals are connected to high and low voltage or to the same voltage.
At commutation (which occurs every sixty degrees of rotation) the motor drive circuit switches motor phases so that a different pair of terminals of the three motor terminals are used for connection to the operating high voltage and the operating low voltage. While commutating the motor and limiting the current with a one-shot (which allows motor current to decay for a fixed amount of time), a problem may occur at a commutation point. If the motor is being driven in any given state by holding one terminal high, one terminal floated, and one terminal pulse width modulated low in order to control current, a problem may occur at a commutation of the high terminal. While commutating to a new high terminal, the old high terminal will continue to pull current from ground through its motor driver. If the one-shot was fired prior to the commutation, the low terminal would be in the high state of its pulse width modulation. The new high terminal will be held high and therefore all the current in the inductance will be dumped into the power supply. This reverse current may cause stability problems for lower cost power supplies. In the past, more expensive power supplies able to withstand the reverse current, or larger bypass capacitors able to absorb the reverse current were used to address this problem. Both approaches result in a more expensive spindle motor system.